The present invention relates broadly to a thermal interface material which is interposable between, for example, the heat transfer surfaces of a heat-generating, electronic component and a thermal dissipation member, such as a heat sink or circuit board, for the conductive cooling of the electronic component. More particularly, the invention relates to a thermally conductive, electrically insulative curable composition, and a cured sheet thereof, which is formulated as including a curable silicone binder and a particulate alumina (Al.sub.2 O.sub.3) filler.
Circuit designs for modem electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like have become increasingly complex. For example, integrated circuits have been manufactured for these and other devices which contain the equivalent of hundreds of thousands of transistors. Although the complexity of the designs has increased, the size of the devices has continued to shrink with improvements in the ability to manufacture smaller electronic components and to pack more of these components in an ever smaller area.
As electronic components have become smaller and more densely packed on integrated boards and chips, designers and manufacturers now are faced with the challenge of how to dissipate the heat which is ohmicly or otherwise generated by these components. Indeed, it is well known that many electronic components, and especially power semiconductor components such as transistors and microprocessors, are more prone to failure or malfunction at high temperatures. Thus, the ability to dissipate heat often is a limiting factor on the performance of the component.
Electronic components within integrated circuit traditionally have been cooled via forced or convective circulation of air within the housing of the device. In this regard, cooling fins have been provided as an integral part of the component package or as separately attached thereto for increasing the surface area of the package exposed to convectively-developed air currents. Electric fans additionally have been employed to increase the volume of air which is circulated within the housing. For high power circuits and the smaller but more densely packed circuits typical of current electronic designs, however, simple air circulation often has been found to be insufficient to adequately cool the circuit components.
Heat dissipation beyond that which is attainable by simple air circulation may be effected by the direct mounting of the electronic component to a thermal dissipation member such as a "cold plate" or other heat sink. The heat sink may be a dedicated, thermally-conductive metal plate, or simply the chassis or circuit board of the device. However, beyond the normal temperature gradients between the electronic component and the heat sink, an appreciable temperature gradient is developed as a thermal interfacial impedance or contact resistance at the interface between the bodies.
That is, and as is described in U.S. Pat. No. 4,869,954, the faying thermal interface surfaces of the component and heat sink typically are irregular, either on a gross or a microscopic scale. When the interfaces surfaces are mated, pockets or void spaces are developed therebetween in which air may become entrapped. These pockets reduce the overall surface area contact within the interface which, in turn, reduces the heat transfer area and the overall efficiency of the heat transfer through the interface. Moreover, as it is well known that air is a relatively poor thermal conductor, the presence of air pockets within the interface reduces the rate of thermal transfer through the interface.
To improve the heat transfer efficiency through the interface, a layer of a thermally-conductive, electrically-insulating material typically is interposed between the heat sink and electronic component to fill in any surface irregularities and eliminate air pockets. Initially employed for this purpose were materials such as silicone grease or wax filled with a thermally-conductive filler such as aluminum oxide. Such materials usually are semi-liquid or solid at normal room temperature, but may liquefy or soften at elevated temperatures to flow and better conform to the irregularities of the interface surfaces.
For example, U.S. Pat. No. 4,299,715 discloses a wax-like, heat-conducting material which is combined with another heat-conducting material, such as a beryllium, zinc, or aluminum oxide powder, to form a mixture for completing a thermally-conductive path from a heated element to a heat sink. A preferred wax-like material is a mixture of ordinary petroleum jelly and a natural or synthetic wax, such as beeswax, palm wax, or mineral wax, which mixture melts or becomes plastic at a temperature above normal room temperature. The material can be excoriated or ablated by marking or rubbing, and adheres to the surface on which it was rubbed. In this regard, the material may be shaped into a rod, bar, or other extensible form which may be carried in a pencil-like dispenser for application.
U.S. Pat. No. 4,466,483 discloses a thermally-conductive, electrically-insulating gasket. The gasket includes a web or tape which is formed of a material which can be impregnated or loaded with an electrically-insulating, heat conducting material. The tape or web functions as a vehicle for holding the meltable material and heat conducting ingredient, if any, in a gasket-like form. For example, a central layer of a solid plastic material may be provided, both sides of which are coated with a meltable mixture of wax, zinc oxide, and a fire retardant.
U.S. Pat. No. 4,473,113 discloses a thermally-conductive, electrically-insulating sheet for application to the surface of an electronic apparatus. The sheet is provided as having a coating on each side thereof a material which changes state from a solid to a liquid within the operating temperature range of the electronic apparatus. The material may be formulated as a meltable mixture of wax and zinc oxide.
U.S. Pat. No. 4,764,845 discloses a thermally-cooled electronic assembly which includes a housing containing electronic components. A heat sink material fills the housing in direct contact with the electronic components for conducting heat therefrom. The heat sink material comprises a paste-like mixture of particulate microcrystalline material such as diamond, boron nitride, or sapphire, and a filler material such as a fluorocarbon or paraffin. The greases and waxes of the aforementioned types heretofore known in the art, however, generally are not self-supporting or otherwise form stable at room temperature and are considered to be messy to apply to the interface surface of the heat sink or electronic component. To provide these materials in the form of a film which often is preferred for ease of handling, a substrate, web, or other carrier must be provided which introduces another interface layer in or between which additional air pockets may be formed. Moreover, use of such materials typically involves hand application or lay-up by the electronics assembler which increases manufacturing costs.
Alternatively, another approach is to substitute a cured, sheet-like material or pad for the silicone grease or wax material. Such materials may be compounded as containing one or more thermally-conductive particulate fillers dispersed within a polymeric binder, and may be provided in the form of cured sheets, tapes, pads, or films. Typical binder materials include silicones, urethanes, thermoplastic rubbers, and other elastomers, with typical fillers including aluminum oxide, magnesium oxide, zinc oxide, boron nitride, and aluminum nitride.
Exemplary of the aforesaid interface materials is an alumina or boron nitride-filled silicone or urethane elastomer which is marketed under the name CHO-THERM.RTM. by the Chomerics Division of Parker-Hannifin Corp., 77 Dragon Court, Woburn, Mass. 01888. Additionally, U.S. Pat. No. 4,869,954 discloses a cured, form-stable, sheet-like, thermally-conductive material for transferring thermal energy. The material is formed of a urethane binder, a curing agent, and one or more thermally conductive fillers.
The fillers, which may include aluminum oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc oxide, range in particle size from about 1-50 microns (0.05-2 mils).
U.S. Pat. No. 4,654,754 discloses a "thermal link" for providing a thermal pathway between a heat source and a heat sink. In one embodiment, a thermally conductive elastomeric material, such as a silicone filled with silver-copper particles, is formed into a mat having a plurality of raised sections. The raised sections deform under low pressure to conform to the space between the heat source and the heat sink.
U.S. Pat. No. 4,782,893 discloses a thermally-conductive, electrically-insulative pad for placement between an electronic component and its support frame. The pad is formed of a high dielectric strength material in which is dispersed diamond powder. In this regard, the diamond powder and a liquid phase of the high dielectric strength material may be mixed and then formed into a film and cured. After the film is formed, a thin layer thereof is removed by chemical etching or the like to expose the tips of the diamond particles. A thin boundary layer of copper or other metal then is bonded to the top and bottom surfaces of the film such that the exposed diamond tips extend into the surfaces to provide pure diamond heat transfer paths across the film. The pad may be joined to the electronic component and the frame with solder or an adhesive.
U.S. Pat. No. 4,842,911 discloses a composite interfacing for the withdrawal and dissipation of heat from an electronic, solid-state device by an associated heat sink. The interfacing consists of dual layers of a compliant silicone rubber carried on either side of a porous glass cloth. The layers are filled with finely-divided heat-conducting particles which may be formed of alumina or another metal oxide, or an electrically-conductive material such as nickel or graphite. One of the silicone layers is pre-vulcanized, with the other being cured and bonded in place once the interfacing has been applied to the heat sink surface for abutment with the electronic device.
Commonly-assigned U.S. Pat. No. 4,869,954 discloses a form-stable material for use in transferring thermal energy from an electronic component to a heat sink. The material is formulated as the reaction product of a urethane resin and a curing agent, and is filled with one or more thermally conductive fillers such as zinc oxide, aluminum oxide, magnesium oxide, aluminum nitride, or boron nitride. The material may be formed as including a support layer of a glass cloth, plastic mesh or film, or a metal mesh or foil.
U.S. Pat. No. 4,965,699 discloses a printed circuit device which includes a memory chip mounted on a printed circuit card. The card is separated from an associated cold plate by a layer of a silicone elastomer which is applied to the surface of the cold plate.
U.S. Pat. No. 4,974,119 discloses a heat sink assembly which includes an electronic component supported on a printed circuit board in a spaced-apart relationship from a heat dispersive member. A thermally-conductive, elastomeric layer is interposed between the board and the electronic component. The elastomeric member may be formed of silicone and preferably includes a filler such as aluminum oxide or boron nitride.
U.S. Pat. No. 4,979,074 discloses a printed circuit board device which includes a circuit board separated from a thermally-conductive plate by a pre-molded sheet of silicone rubber. The sheet may be loaded with a filler such as alumina or boron nitride.
U.S. Pat. No. 5,060,114 discloses a conformable, gel-like pad having a thermally-conductive filler for conducting heat away from a packaged electronic power device. The pad is formed of a cured silicone resin which is filled with a thermally-conductive material such as aluminum powder, nickel, aluminum oxide, iron oxide, beryllium oxide, or silver. A thin sheet of a thermally-conductive metal such as aluminum is positioned in contact with the surface of the conformable pad for increased thermal transfer.
Commonly-assigned U.S. Pat. No. 5,137,959 discloses a thermally-conductive, electrically insulating interface material comprising a thermoplastic or cross linked elastomer filled with hexagonal boron nitride or alumina. The material may be formed as a mixture of the elastomer and filler, which mixture then may be cast or molded into a sheet or other form.
U.S. Pat. No. 5,151,777 discloses an interface device of thermally coupling an integrated circuit to a heat sink. The device includes a first material, such as copper, having a high thermal conductivity, which is provided to completely surround a plurality of inner core regions. The inner core regions contain a material such as an iron-nickel alloy having a low coefficient of thermal expansion.
Commonly-assigned U.S. Pat. No. 5,194,480 discloses another thermally-conductive, electrically-insulating filled elastomer. A preferred filler is hexagonal boron nitride. The filled elastomer may be formed into blocks, sheets, or films using conventional methods.
Commonly-assigned U.S. Pat. Nos. 5,213,868 and 5,298,791 disclose a thermally-conductive interface material formed of a polymeric binder and one or more thermally-conductive fillers. The fillers may be particulate solids, such as aluminum oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc oxide. The material may be formed by casting or molding, and preferably is provided as a laminated acrylic pressure sensitive adhesive (PSA) tape. At least one surface of the tape is provided as having channels or through-holes formed therein for the removal of air from between that surface and the surface of a substrate such as a heat sink or an electronic component.
U.S. Pat. No. 5,309,320 discloses a "conduction converter" for a printed circuit board having electronic components. The converter includes a body of a thermally conductive dielectric material, such as an alumina-filled RTV silicone, which is molded to the exact configuration of the electronic components. The converter may be clamped intermediate a cold plate and the circuit board to conductively remove heat from the electronic components.
U.S. Pat. No. 5,321,582 discloses an electronic component heat sink assembly which includes a thermally-conductive laminate formed of polyamide which underlies a layer of a boron nitride-filled silicone. The laminate is interposed between the electronic component and the housing of the assembly.
Commonly-assigned U.S. Pat. No. 5,510,174 discloses a thermally-conductive, titanium diboride (TiB.sub.2) filler providing improved thermal conductivity at low application pressures. The filler may be incorporated into elastomers, films, and tapes.
U.S. Pat. No. 5,545,473 discloses a thermally conductive interface for electronic components. The interface is formed of an open structure fluoropolymer material such as an expanded polytetrafluoroethylene. Thermally conductive particles, which may be formed of a metal or metal oxide, or another material such as boron nitride, aluminum nitride, diamond powder, or silicone carbide, are attached to portions of the fluoropolymer material.
Sheets or pads of the above-described types have garnered general acceptance for use as interface materials in the conductive cooling of electronic component assemblies. It will be appreciated, however, that further improvements in these types of interface materials would be well-received by the electronics industry. Especially desired would be a low cost alumina-filled silicone material having thermal conductive and electrical insulating properties approaching those of higher cost boron nitride filled materials.